The present invention relates generally to aerospace power supplies as used in flying/swimming/crawling systems and, more particularly, to a method and apparatus for monitoring two or more buses providing power to such a system and performing a current zero-crossing switch from a faulty bus to a good bus prior to failure of the faulty bus and with no power interruption for the system. Any failure of power supplied on a bus such that the voltage or frequency of the power is in error by more than a given percentage, for example .+-.5%, will be referred to as a fault or a faulty bus herein.
Aerospace power supplies are used to power important operational systems in a variety of flying/swimming/crawling applications. For example and perhaps the most critical systems are in aircraft and include the avionics and flight control systems. If the operation of these systems are interrupted by a power failure, the time required to restart the systems can be much more than an inconvenience to the operators.
To ensure continuous operation, aerospace power supplies normally have redundant power buses, typically comprising two or more three phase buses, which are switchably interconnected to the systems which they power. While redundant power buses held to ensure a reliable source of power, in the event of failure of a currently connected bus, the switching operation can itself lead to a power interruption which requires a system powered thereby to be restarted. To overcome such switching problems and to maintain the voltage level and frequency of an ac power supply to a given system, ac-to-ac converters have been used to interface between ac power supplies and the loads which they power.
When ac-to-ac converters are used, the importance of the voltage level and frequency of the primary supply is substantially reduced. Typically, ac-to-ac converters receive ac power from a primary supply, such as a generator, convert the ac power to dc power and then "construct" or generate ac power of the required voltage level and frequency, for example 110 volts and 400 hertz, for in turn powering the aerospace equipment. Thus, as long as sufficient power is available to maintain the dc power at a level capable of being converted into the required ac output power, the voltage level and frequency of the input ac power is not critical. Further, the ac-to-ac converters provide carryover and buffer switches between power buses in the event a primary ac power supply fails. While ac-to-ac converters are thus attractive as providing closely regulated power, both from a voltage level and frequency standpoint, such converters are expensive and tend to be relatively heavy.
Currently available regulators make possible the generation of ac power which can be maintained within tolerable limits for voltage level and frequency. However, maintaining continuous power for an aerospace load while switching from one bus to another still creates problems. If the switch is to performed as a "make-before-break" switch, the two buses are connected together and accordingly must be synchronized with one another or be capable of withstanding substantial transient power levels of not synchronized. On the other hand, if the switch is to be performed as a "break-before-make" switch, while synchronization is not required, provision must be made for carryover of the power provided to the load and the power supplies must again be able to withstand substantial transient power levels. Once again, appropriate systems are both expensive and tend to be heavy.
It is thus apparent that a need exists for an improved arrangement for switching between or among two or more power buses to provide substantially continuous power to critical aerospace loads without having to synchronize the power buses and without generating substantial transients which can damage the power buses and potentially interfere with the operation of the aerospace loads.